1. Field of the Invention
This application is related to stabilizing the frequency of a tunable laser and, more particularly, to using spectral hole burning materials to stabilize the frequency of a tunable laser.
2. Description of the Related Art
Lasers emit electromagnetic radiation characterized by the optical range of the spectrum where wavelengths are expressed in nanometers (nm) corresponding to 10xe2x88x929 m, and frequencies are expressed in megaHertz (MHz) corresponding to 106 Hz, or gigaHertz (GHz) corresponding to 109 Hz, or teraHertz (THz) corresponding to 1012 Hz, where one Hz is one cycle per second.
Previous laser stabilization techniques have relied on frequency references based on Fabry-Perot interferometer resonances or the center frequencies of transitions in atomic or molecular vapors.
In the prior art, for example, the Pound-Drever-Hall laser frequency locking technique (R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, Laser Phase and Frequency Stabilization Using an Optical Resonator, Appl. Phys. B31, 97 (1983), M. Zhu and J. L. Hall, Stabilization of optical phase/frequency of a laser system: application to a commercial dye laser with an external stabilizer, J. Opt. Soc. Am. B 10, 802 (1993)) utilizes frequency modulated (FM) spectroscopy techniques (G. C. Bjorklund, M. D. Levenson, W. Lenth, C. Ortiz, Frequency Modulation (FM) Spectroscopy: Theory of Lineshapes and Signal-to-Noise Analysis, Appl. Phys. B 32, 145 (1983)) to actively lock the laser frequency to a reflection mode of a Fabry-Perot interferometer.
Use of an error signal to stabilize a laser frequency of a tunable laser according to the Pound-Drever-Hall technique is shown in FIG. 1. In FIG. 1A a laser 110 emits a beam along path 161 impinging on a phase modulator 112 The radio frequency (rf) signal generator 142 supplies an electrical signal transmitted by wire to the modulator 112 and to mixer 140. The modulator 112 phase modulates and passes a first beam portion along path 162 to a beam splitter 114. Beam splitter 114 passes a second beam portion along a beam path 163 to a cavity 116 between mirrors 115 and 117. The cavity 116 is tuned to a reference frequency by choice of spacing between mirrors 115 and 117 and the cavity emits a reference beam having an energy peak at the reference frequency along path 163 to splitter 114. At beam splitter 114 both the modulated beam, which reflects immediately off mirror 115, and the reference beam are diverted along path 165 to a detector 120 which outputs a detector electrical signal. The detector reference signal contains rf frequency components generated by the interference between the modulated beam and the cavity reference beam. This detector electrical signal is filtered at filter 130 to remove unwanted harmonic frequencies and to generate an error signal that is output to the mixer 140. At mixer 140 the rf signal from the rf signal generator 142 is combined with the error signal at rf frequencies to demodulate the error signal to lower frequencies where it is used as a control signal that is output to the laser servo electronics 150. The servo electronics 150 tune the laser 110 in response to the control signal received.
FIG. 1B shows the output 125 of cavity transmission detector B 122 and the error signal 145 output by error signal detector A 120 combined with the mixer 140 which is sent to the servo electronics 150 as the modulated laser frequency beam 162 is tuned across the cavity reference frequency. The amplitude of the error signal 145 at a given frequency in the vicinity of the lock point 147 is related to the size and direction of the deviation of frequency from the reference frequency at the lock point 147. Thus the greater the deviation of the laser frequency from the reference frequency, the greater is the error signal and the greater is a component of the input to the servo electronics 150 to tune the laser back toward the reference frequency at the lock point 147. The stability of the laser frequency is limited by the characteristics of the peak in the cavity transmission signal used as the reference frequency detected by the detector 120.
To evaluate locking stability, two lasers are locked to adjacent modes of the same cavity. The beat signal between the two laser frequencies is then monitored for fluctuations. A relative locking performance of 1 part in 105 of the interferometer linewidth has been recently demonstrated (G. Ruoso, R. Storz, S. Seel, S. Schiller, J. Mlynek, Nd: YAG laser frequency stabilization to a supercavity at the 0.1 Hz level, Opt. Comm. 133, 259 (1997)) with a high finesse interferometer cavity having a resonance linewidth of less than 10 kHz, leading to relative stability between two lasers at the sub-Hz level for nominally a minute. The short-term drift of the frequency of either of these lasers arises from thermal length changes, mechanical creep, vibration, or other variation in the length of the reference cavity and hence resonance frequency. A recent notable attempt to approach absolute stability used Fabry-Perot cavities made from sapphire and kept at cryogenic temperatures to achieve a 3 kHz drift over 6 months (R. Storz, C. Braxmaier, K. Jxc3xa4ck, O. Pradl, S. Schiller, Ultrahigh long-term dimensional stability of a sapphire cryogenic optical resonator, Opt. Lett. 23, 1031 (1998)).
Atomic transitions are commonly used to lock a laser to a specific, absolute frequency. A typical arrangement would substitute a fixed atomic resonance for the cavity resonance and allow the modulated beam to transmit through a gas-phase sample before collecting it on a photodetector. Today""s precision clocks and oscillators are based on well-studied microwave transition frequencies of rubidium, cesium, hydrogen and mercury atoms in oscillators and masers. Optical frequency standards in the communications bands of 1.3 and 1.5 micron wavelength are also of interest. Among the principal advantages and difficulties with existing microwave and optical standards are that the transition frequencies of an atom are unique to its type, i.e. predetermined by nature, with consequent limitations on the choice of frequency, and that FM locking techniques typically select the center of those transitions. If the frequencies are not exactly those of interest, they must be transferred through an elaborate chain of precisely controlled optical and radio frequency (RF) synthesis, such as frequency doubling and mixing or parametric oscillation sums and differences. Lastly, strong optical dipole transitions typically have spectral linewidths in the 10""s of MHz (megaHertz, i.e., 106 Hz)xe2x80x94not especially narrow for use as a precise frequency discriminator for laser locking, though examples of higher multi-pole moment transitions with narrower linewidths do exist.
Another frequency locking technique involves stabilization to a Lamb dip in a gas vapor cell. For the Lamb dip, the gaseous motion of the atoms produces a Doppler shifted distribution of frequencies creating an inhomogeneously broadened absorption-line. A strong laser, intense enough to saturate a particular velocity subset of these atoms can temporarily create a spectral hole anywhere in the absorption profile until it is turned off and the hole disappears (absorption reappears). A less intense probe laser can be locked to the hole created in the presence of the strong laser. The frequency stability of the Lamb dip spectral hole is determined by that of the strong laser. To remove this external stability dependence, the more typical arrangement is to use a single laser divided into two counter-propagating beams, strong and weak. For a lock to occur, the two beams must interact with the same velocity subset of atoms that must be the non-Doppler shifted subset. The lock is then constrained to the center of the inhomogeneous absorption.
Condensed phase spectral hole burning materials are known. Absorption features of ions or molecules doped into condensed phase materials are spectrally broadened by two main classes of mechanisms. Homogeneous broadening is the fundamental broadening experienced by all ions or molecules independently, and arises from the quantum-mechanical relationship between the lineshape and the dephasing time of the excited ion. At cryogenic temperatures, such homogeneous linewidths have been measured, using the photon-echo technique, to be as narrow as 100 Hz or less, orders of magnitude sharper than most gas phase transitions (R. W. Equall, Y. Sun, R. L. Cone, R. M. Macfarlane, Ultraslow Optical Dephasing in Eu3+:Y2SiO5, Phys. Rev. Lett. 72, 2179 (1994)). Inhomogeneous broadening, such as that of the inhomogeneous absorption profile 210 depicted in FIG. 2A, arises from the overlap of the quasi-continuum of individual spectra 220 of all of the ions or molecules in the condensed phase material, which have microscopically different environments and therefore slightly different transition frequencies. The extent of this envelope can be anywhere from hundreds of MHz to the THz (teraHertz, i.e., millions of MHz) range.
Spectral hole burning (W. E. Moerner, ed., Persistent Spectral Hole Burning: Science and Applications, (Springer-Verlag, Berlin 1988); R. M. Macfarlane and R. M. Shelby, xe2x80x9cCoherent Transient and Spectral Holeburning Spectroscopy of Rare Earth Ions in Solids,xe2x80x9d in Spectroscopy of Solids Containing Rare Earth Tons, A. A. Kaplyanskii and R. M. Macfarlane, eds. (North Holland, Amsterdam 1987)) uses a narrow-band laser to selectively excite only the small fraction of ions or molecules whose frequencies around a central frequency 240 coincide with that of the laser. If some mechanism exists to remove those ions or molecules from the absorbing population, or to change their resonant frequencies, then the inhomogeneous absorption profile can be temporarily or permanently altered, leaving a xe2x80x9cspectral holexe2x80x9d 250 at the frequency of the laser, as seen in FIG. 2B depicting the altered inhomogeneous absorption profile 230. In the cases of interest here, the homogeneous linewidth 220xe2x80x2xe2x80x3 is many orders of magnitude smaller than the inhomogeneous linewidth 210, and by probing with a tunable laser in the vicinity of a spectral hole, a sharp positive transmission peak is observed on a broad background of partial transmission. A very flexible advantage of hole burning, in contrast to isolated atomic transitions, is that the center frequency 240 for the hole may be chosen anywhere within the wide inhomogeneous band of absorbing frequencies. Furthermore, multiple holes may be placed within one inhomogeneously broadened absorption band.
Mechanisms exist to provide permanent change to preserve the hole, the most common being (a) excitation-induced changes in the lattice near the optically active ion or molecule, (b) photoionization of that ion or molecule itself, and (c) photochemical reactions. Possibilities exist to use photon-gated processes where a second, possibly broadband, light source is required to produce a persistent hole. With such a process, a single probe laser can be used to read the hole without altering it through further hole burning.
As described above, the frequency locking achievable using Fabry Perot cavities, as shown in FIG. 1A, is limited by the thermal length changes, mechanical creep, vibration and other variations in the length of the reference cavity influenced by external causes. As also described above, the frequency locking achievable using atomic transitions are limited by available central frequencies associated with those transitions and the characteristics of the frequency reference such as the Lamb dip. Thus there is a need for laser frequency stabilization techniques capable of achieving smaller variations in frequency for longer periods of time at more flexibly selected frequencies than are available in the prior art.
An object of the present invention is to link together the existing techniques of Pound-Drever-Hall type laser frequency locking and persistent spectral hole burning in condensed phase materials to create a laser frequency locking scheme with the potential to stabilize the frequency of a laser on a longer time scale and with a greater accuracy than with existing techniques. The processes for producing each of these techniques are specialized subjects, and the two have not been previously combined.
In the prior art Pound-Hall-Drever laser frequency locking scheme, the resonant modes of a high-finesse Fabry-Perot interferometer are used as a frequency reference for stabilizing a laser. One embodiment of the present invention, however, uses a preprogrammed spectral hole in the absorption profile of a spectral hole burning material (SHBM). As with locking to a gas-phase atomic transition, this provides the potential for high stability over long time periods since the resonance of an ion or molecule in a condensed phase material is fixed for a known temperature. Furthermore, with spectral hole burning done at cryogenic temperatures, the potential exists for extremely sharp resonances.
Unlike with gas phase transitions, locking frequencies can be chosen anywhere within a much broader inhomogeneous absorption profile. In addition, several lasers can be stabilized to multiple spectral holes within the same SHBM with arbitrary frequency separations. This gives rise to programmable and extremely stable beat frequencies between these lasers for such applications as optical clocks and programmable optical waveform generators.
Persistent spectral hole burning offers distinct advantages over the textbook technique of locking to a Lamb dip in a gas vapor cell. The saturation of vapor is only transient, and the lock point is constrained to the center of the broad inhomogeneous absorption to maintain stability. The primary differences of a persistent spectral hole-burning material are that the holes are potentially permanent and very narrow, the hole frequencies are fixed without requiring the continuing stability of the burning laser, and the locking frequencies are not limited to line-center. Further advantages of frequency locking to a persistent spectral hole are: insensitivity to vibrations (which affect the spacings and frequencies of Fabry Perot cavities); insensitivity to thermal variations of the environment obtained through the high degree of thermal isolation inside a cryostat, an asymptotically vanishing dependence of the hole frequency on temperature as a solid material is chilled nearer to absolute zero, and the ability to produce a compact and transportable setup that does not require the ultra-high vacuum equipment necessitated for atomic ion trap frequency standards.
To achieve the foregoing and other objects and advantages of the present invention, as embodied and broadly described herein, techniques for stabilizing a laser at a selectable frequency include splitting an output beam from an electrically adjustable laser into a first beam and a second beam. The second beam is transmitted through a modulator to produce a modulated second beam. Then the modulated second beam is transmitted through a spectral hole burning material onto a detector. The laser is electronically adjusted in response to a detector output from the detector which senses the changes in the modulated second beam after it passes through the spectral hole burning material.
According to another aspect of the invention, techniques for producing light pulses at a beat frequency include producing a first frequency stabilized laser beam at a first frequency and producing a second frequency stabilized laser beam at a second frequency different from the first frequency by an amount related to the beat frequency. Producing the first frequency stabilized laser beam involves transmitting a first output beam from an electronically adjustable first laser through a first spectral hole burning material with a spectral hole at the first frequency onto a first detector. The first laser is electronically adjusted based on a first frequency difference between the first output beam and the first spectral hole, said first frequency differenceanalyzed from a first detector output from the first detector. Producing the second frequency stabilized laser beam at a second frequency involves transmitting a second output beam from an electronically adjustable second laser through a second spectral hole burning material with a spectral hole at the second frequency onto a second detector. The second laser is electronically adjusted based on a second frequency difference between the second output beam and the second spectral hole, said second frequency difference analyzed from a second detector output from the second detector. The first and second frequency stabilized laser beams are merged to interfere with each other.
In another embodiment of the invention, the first spectral hole burning material is identical to the second spectral hole burning material. In this case the spectral hole burning material has spectral holes at both the first frequency and the second frequency.
In another aspect of the invention, a spectral hole burning material for use in stabilizing a laser frequency at a target frequency with a target laser line width, has a set of homogeneous absorption lines. A particular line of the set of homogeneous absorption lines has a particular center frequency at the target frequency.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.